1. Field of the Invention
The present invention relates in general to wireless communications and in particular to circuits, systems and methods for correction of phases and magnitude errors im image reject mixers.
2. Description of the Related Art
In conventional wireless communications systems, data is transmitted by modulating a carrier signal at a frequency f.sub.c using any one of a number of well known modulation techniques, for example phase shift keying or frequency shift keying. This signal typically represents a single channel in a multiple channel frequency band allocated for the particular application or purpose. After receipt by the receiver antenna, the modulated signal is typically passed through a low noise amplifier to improve the overall receiver noise figure and then through a front end filter. The signal next is either upconverted or downconverted using one or more mixing stages to a desired frequency which can more easily be processed. In a digital receiver, this processing includes analog to digital conversion following any analog processing, such as analog filtering.
The mixing taking place during up- or down-conversion mixes the received signal with a local oscillator signal at a desired local oscillator frequency f.sub.lo. The result is multiple intermediate frequency (IF) signals, for example one at f.sub.c +f.sub.lo and another at f.sub.c -f.sub.lo. One of these signals is the desired signal and the other is its image. The image, along with f.sub.lo and various subharmonics resulting from the mixing are filtered out, for example using preselection filtering. While this type of system if suitable for narrow band applications, it is not practical in wide band applications where the image could fall in the band of interest and appear as a legitimate signal.
In some wideband applications, image rejection mixers are commonly used. In an image rejection mixer, the incoming signal is mixed with the local oscillator signal and in parallel with the local oscillator signal shifted by ninety (90) degrees. The resulting in-phase (I) and quadrature (Q) signals together represent a complex signal (with I the real part and Q the complex part) which is for further processing (in a digital receiver these signals are normally sent on a pair of analog to digital converters (ADCs)).
Image rejection mixers have some serious limitations. Among other things, to achieve acceptable levels of image rejection, the pair of mixers must be closely matched in output amplitude and phase. Even a very small mismatch of 1% or less in amplitude or phase across the entire band can cause the image amplitude to rise significantly within the complex signal. This degree of matching is impractical to implement in most systems.
Some current image rejection schemes down-convert each channel in the given frequency band to baseband on an individual basis. Magnitude and phase errors between the I and Q components can then be corrected at lower frequencies where filter matching is less critical. This however requires one or more variable frequency local oscillators and corresponding loop filters, at either the RF or IF conversion stages, for mixing each channel of interest to baseband. In other words, in these systems, the entire band of interest (containing many potential channels) cannot be converted with a fixed local oscillator while maintaining satisfactory image rejection.
Therefore, a need has arisen for circuits, systems and methods for maintaining high image rejection in broadband wireless applications.